Refer to the Template Hierarchy article in the WordPress Codex. If you read and digest that whole article you will be able to figure it out.

You can also log into the back-end and edit that specific page and see if a template is defined in the box on the right, but likely it is a custom post type archive template so that probably won't get you anywhere.

It depends a lot on how the site is set up, but you are probably looking for a template file starting with archive or category or taxonomy as that is definitely an archive page of some kind. If you can provide further insight into how the content is organized (custom post types, custom taxonomies, tags, categories, and so forth) I can help further.